1. Field of the Invention
The present invention relates to an optical element for use in a display such as a liquid crystal display, more particularly, a circularly polarizing element including cholesteric liquid crystal layers, and to a process for producing the same. The term “liquid crystal layer” as used herein means a layer having the nature of liquid crystal in an optical sense, and includes not only a layer of a liquid crystalline phase having flowability but also a layer of a solid phase obtainable by solidifying a liquid crystalline phase with the orientation of molecules in the liquid crystalline phase maintained.
2. Description of Related Art
There has conventionally been known a circularly polarizing element including a cholesteric liquid crystal layer, capable of reflecting either right- or left-handed circularly polarized component of light of a wavelength equal to the pitch of a helix formed by liquid crystalline molecules in the liquid crystal layer and transmitting the other circularly polarized component of the light. In such a circularly polarizing element, a method in which the helical pitch in the single liquid crystal layer is continuously varied in the direction of thickness has been known as a technique for broadening the selective reflection wave range that is covered by the circularly polarized element (Japanese Laid-Open Patent Publication No. 281814/1994, etc.). Other known methods useful for this purpose are as follows: a method in which a plurality of liquid crystal layers having different helical pitches are laminated (Japanese Patent Application No. 56916/2001); and a method in which, after laminating two cholesteric liquid crystalline polymer layers by contact bonding, the helical pitches in the layers are varied stepwise through heat treatment (Japanese Laid-Open Patent Publication No. 44816/1999).
In the above-described circularly polarizing element, in order to make the reflectance of the circularly polarizing element for circularly polarized light high, it is necessary to make the liquid crystal layer sufficiently thick. It is noted that there is a definite relationship between the reflectance of a liquid crystal layer for circularly polarized light and the thickness of the liquid crystal layer. For example, in the reflection of circularly polarized green light (central wavelength: 520 nm) by a cholesteric liquid crystal layer with a refractive index n of 1.56, the relationship between the reflectance of the liquid crystal layer for the circularly polarized light and the thickness of the liquid crystal layer is as shown in FIG. 6. As can be understood from this figure, in order to make the reflectance of the liquid crystal layer for the circularly polarized light as high as 90% or more, the liquid crystal layer is required to have a thickness of approximately 2.5 μm.
To make a cholesteric liquid crystal layer reflect approximately 90% of circularly polarized light of a certain wavelength, it is, in general, necessary that 7 to 8 helical pitches be present in the liquid crystal layer in the direction of thickness. One pitch corresponds to the distance it takes for liquid crystalline molecules to rotate through 360°. Therefore, the thickness required for a liquid crystal layer to reflect approximately 90% of circularly polarized light of a certain wavelength is given by the following Eq. (1), using the wavelength of the circularly polarized light and the refractive index of the cholesteric liquid crystal layer:(Thickness of liquid crystal layer)==8 pitches×(Wavelength of circularly polarized light) ÷(Refractive index of cholesteric liquid crystal layer)  (1)
In the case where a circularly polarizing element having a high reflectance for circularly polarized light over the whole visible light range (e.g., 400–700 nm) is obtained by using, as a technique for broadening selective reflection wave range, a method in which the helical pitch in a single liquid crystal layer is continuously varied in the direction of thickness, it is necessary to make the thickness of the liquid crystal layer several tens micrometers (e.g., 20μm or more). In general, if a liquid crystal layer has a surface area of (100 mm×100 mm) or more, it is necessary to make its thickness approximately 10μm, more preferably 5 μm or less, in order to ensure good alignment of liquid crystalline molecules over the entire surface of the liquid crystal layer with such a large surface area. This is because if the liquid crystal layer is too thick, it is apt to have an alignment defect called “oily streak” and to suffer from focal conic. If such troubles are caused, the liquid crystal layer becomes opaque, for example, and its optical properties thus remarkably deteriorate.
On the other hand, in the case where a circularly polarizing element is obtained by using, as a technique for broadening selective reflection wave range, a method in which a plurality of liquid crystal layers having different helical pitches are laminated, that is, a method in which a plurality of liquid crystal layers respectively covering any narrow selective reflection wave ranges on the short to long wavelength side, are laminated, each liquid crystal layer is made to have a thickness of several micrometers. It is, however, necessary to make those liquid crystal layers that cover wave ranges on the longer wavelength side thicker, so that above-described troubles tend to occur. Moreover, in this method, the compositions of coating liquids for forming the liquid crystal layers are different, and the conditions under which the liquid crystal layers are formed are also different. Therefore, it is not easy to stably control the reflectance/transmittance of the resulting circularly polarizing element for circularly polarized light.